1. Technical Field
This invention relates generally to a communication system for a single-wire interface, and more particularly to a communication system capable of communicating between, for example, a thermostat and a receiving unit disposed near or in an air compressor by way of high frequency current modulation along a single HVAC control wire.
2. Background Art
As the cost of energy continues to rise, heating and cooling a home has become a complicated activity. When natural gas, heating oil and electric power were plentiful and inexpensive, one may simply have set the thermostat on 78 in the summer and 68 in the winter to adequately heat and cool a house. Under such a plan, they may only touch the thermostat twice in a year.
With the advent of new technology, combined with rising energy costs, it is often financially advantageous to become a more active participant in the heating and cooling of the home. For instance, utilities, in an effort to shave demand peaks and otherwise smooth demand, may offer customers variable rate plans. Under these variable rate plans, a consumer may pay A cents per unit for energy at 10 AM, B cents per unit at 2 PM, and C cents per unit at 11 PM. Further, some utilities offer cost advantages to consumers who allow the energy provider to override their programmed thermostat settings at peak demand times to help prevent brownouts and blackouts.
These new pricing and control programs necessitate a communication link between the energy provider and the consumer's HVAC system, particularly the thermostat. This need for a communication link to the interior of a consumer's home presents two problems: first, traditional thermostats that use bimetal temperature sensors and mercury switches are incapable of accommodating digital communication. Second, a traditional heating, ventilation and air conditioning (HVAC) system includes only a few control wires. Conventional HVAC systems have only four wires running from the load devices, like the air compressor, furnace and air handler, to the thermostat. One wire is used for cooling control, one for heating control, one for fan control and one supplying an electrically isolated, 24-volt, class-II connection to the other three wires when the switches in the thermostat are closed. As such, even where a mechanical thermostat is replaced with an electronic one having a microprocessor capable of communicating with other devices, there is no suitable communication bus with which to connect an exterior data device with the thermostat.
One solution to this lack of a communication bus is to rewire a building with communication cables running from outside the building directly to the thermostat. This solution, however, is both time consuming and expensive. A technician must drill holes, fish cables, and install new power sources. Often this installation can be cost prohibitive for consumers.
An alternate solution is to equip a thermostat with a wireless communication system. The problem with this solution is that such a wireless connection requires more power than can be sourced by the 24-volt wire running to the thermostat. Consequently, additional wiring must still be provided to supply power to the communication device. Again, installation of additional wiring into existing structures may be cost prohibitive. While a battery may be used to power the wireless communication system, the user must take care to ensure that the batteries are continually replaced, which is inconvenient and costly. Further complicating matters, reception problems may exist with wireless systems due to interior walls and signal multipaths.
There is thus a need for an improved communication system suitable for retrofitting into conventional HVAC systems that both requires no additional wiring and is capable of operating from the 24-volt power wire without adversely affecting the operation of the HVAC system.